The background to the present invention is set out in the specification of prior PCT patent application No. PCT/AU89/00254 filed in the name of the of the present applicant (herein called `our prior application`). Reference should be made to the specification of that application for a discussion of the technical problems faced in the design and operation of fading simulators and for an explanation of the applicant's desiderata in designing such devices.
The simulator of our prior application allowed microprocessor-controlled calibration of notch depth and notch position as independent variables thereby greatly reducing calibration time. Since the drift of component parameters normally requires frequent re-calibration of the simulator during use, the significant reduction in calibration time offered by the simulator design and method of our prior application provided a substantial advantage with respect to the art at the time it was filed. Nevertheless, calibration of that simulator could still take a few minutes and it would clearly be desirable if such calibrations were to take much less time while still allowing accurate and predictable simulation of deep signal fades which sweep rapidly across the signal spectrum.
U.S. Pat. No. 5,062,148 to Edwards discloses a multipath fading simulator in which each path includes a 180.degree. phase shifter and an attenuator which are jointly controlled so that each phase shifter contributes to the total difference in phase between the paths and so that each attenuator contributes to the required signal level difference between the paths. By closely matching the characteristics of the two phase shifters, compensation for parameter drift and errors in the phase shifters can be achieved. Similarly, by matching the attenuators, compensation for drift and errors can be achieved. However, calibration is complicated and calibration time is increased.